1. Field of the Invention
This invention relates to a prepit detection unit for detecting prepits in an optical recording and reproducing apparatus for recording and reproducing information on a recordable, optical record medium including preinformation such as address information in a prepit form.
2. Description of the Related Art
Generally, address information and reference signals for generating clock signals used for the recording operation and the reproducing operation are previously recorded on a recordable, optical record medium in a prepit or pregroove form to enable information to be recorded on the optical record medium in an unrecorded state. For example, in each pregroove of an area in which the information to be essentially recorded, such as video data or audio data, is recorded as record pits and a land portion of an area between the pregrooves, prepits, which will be hereinafter referred to as land prepits (LPP), are recorded on DVD-R (Digital Versatile Discxe2x80x94Recordable) actively developed in recent years as a record medium capable of recording information about seven times larger than a CD (Compact Disc). The LPP is formed on a line perpendicular to the tangential direction of the pregroove so as not to exist in the adjacent land with the pregroove between.
To detect an LPP, reflected light of a light beam applied to a pregroove is received at a light reception element divided into two parts by a division line optically parallel to the tangential direction of at least the pregroove and the difference between output signals from the areas of the light reception element (divisional portions) in the direction perpendicular to the pregroove is calculated, then the LPP is detected as a binary signal provided by comparing the differential signal with a predetermined threshold value (the binary signal will be hereinafter referred to as LPP signal) More particularly, as shown in FIG. 4, reflected light from DVD-R (not shown) is received at light reception means divided into a first divisional light reception part la and a second divisional light reception part 1b by a division line 1 optically parallel to the tangential direction of an information track of the DVD-R. First and second light reception signals output from the first and second divisional light reception parts 1a and 1b are amplified by amplifiers 2 and 3 respectively and are input to a radial push-pull generation circuit 4 made of a differential amplifier. If the record medium is an optical disc, the divisional areas of the light reception element are formed along the radial direction of the disc by the above-mentioned division line and therefore the differential signal is called a radial push-pull signal.
The reason why an LPP can be thus detected based on the radial push-pull signal is that the LPP is formed so as not to exist in the adjacent land on the line perpendicular to the tangential direction of the pregroove, as described above. That is, when a light beam is applied to one pregroove, the reflected light from the lands on both sides of the pregroove does not contain an LPP reflection component at the same time (LPP component is contained only in the reflected light from either of the lands) and thus only the reflected light component of the LPP is extracted by calculating the difference as mentioned above. Usually, however, only either of the both polar components (for example, positive polar component) is extracted by calculating the difference.
The provided differential signal is further input to an LPP detector 5 and is compared with a predetermined threshold value. The binary signal resulting from comparing the differential signal with the threshold value is the LPP signal.
If the environment in which a recording and reproducing apparatus for recording and reproducing information on a record medium, comprising the prepit detection unit changes and optical axis fluctuation occurs in a laser oscillator or a pickup contains optical aberration, the light quantity of the light beam output by the pickup goes out of balance. The light quantity of reflected light from DVD-R incident on the light reception element may get out of balance in the radial direction because the reflected light quantity goes out of balance because of the inner and outer periphery difference in reflection factor of the disc in addition to the above-described cause during recording onto the record medium.
In this case, the record pit component may not completely be canceled simply by generating the radial push-pull signal of a reproduction signal. Resultantly, a differential signal occurs in the push-pull signal of a read signal at a place where LPP does not essentially exist on the record medium, and a pseudo-LPP signal as if LPP existed at the place appears as radial push-pull signal output. Resultantly, a CPU recognizes the pseudo-signal as false preformat data and uses it for address retrieval and machine control, resulting in malfunction of the whole apparatus.
More particularly, during recording onto a record medium, RF signals provided in the first divisional light reception area 1a and the second divisional light reception area 1b ideally become signal waveforms as shown in FIGS. 7A and 7B. That is, the RF signals A and B have signal waveforms of the same amplitude on the same reference potential (xe2x80x9c0xe2x80x9d in FIGS. 7A and 7B) in other than the LPP existence portion, and LPP components appear in the mutually inverted polarity directions because of the LPP in the LPP existence portion. When the RF signals A and B pass through the radial push-pull generation circuit 4, a differential signal shown in FIG. 7C is provided, and is compared with a predetermined threshold value (horizontal dotted line shown in FIG. 7C) by the LPP detection circuit 5, whereby an LPP signal is extracted. However, if an offset occurs relative to the reference potential for the RF signals provided in the first divisional light reception area 1a and the second divisional light reception area 1b, and the RF signals differ in amplitude like signal waveforms shown in FIGS. 7D and 7E because of the above-described causes, etc., the differential signal provided by the radial push-pull generation circuit 4 has a value not essentially existing because of the offset potential and RF signal amplitude differences as shown in FIG. 7F. If the differential signal F is compared with a predetermined threshold value (horizontal dotted line shown in FIG. 7F) by the LPP detector 5, an erroneous LPP signal is extracted.
It is therefore an object of the invention to provide a prepit detection unit capable of extracting an LPP signal accurately in such a record medium with variations in reflected light quantity in the radial direction or an apparatus for recording and reproducing information on a record medium.
To achieve the above object, according to the invention, there is provided a prepit detection unit for applying a light beam to an information record track of an optical record medium comprising the information record track for recording record information and a guide track for guiding the light beam into the information record track with a prepit for holding preinformation, formed on the guide track, receiving reflected light of the applied light beam at light reception means being divided into a first divisional light reception part and a second divisional light reception part by a division line optically parallel to a tangential direction of the information record track, calculating a difference between a first read signal output from the first divisional light reception part and a second read signal output from the second divisional light reception part by a differential device, and detecting the prepit based on a differential signal output from the differential device, wherein the differential device comprises first signal potential correction means for causing a recorded portion potential of the first read signal and a recorded portion potential of the second read signal to match a first reference potential, and second signal potential correction means for causing an unrecorded portion potential of the first read signal and an unrecorded portion potential of the second read signal to match a second reference potential, and calculates the difference between the first read signal and the second read signal corrected by the first and second signal potential correction means.